Knife sharpener



FIG. 1 is a first perspective view of a knife sharpener showing my new design;

FIG. 2 is a second perspective view thereof;

FIG. 3 is a front elevation view thereof;

FIG. 4 is a rear elevation view thereof;

FIG. 5 is a left side elevation view thereof

FIG. 6 is a right side elevation view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof.

The broken lines shown in the drawings depict portions of the knife sharpener in which the design is embodied that form no part of the claimed design. 

CLAIM The ornamental design for a knife sharpener, as shown and described. 